(1) Field of the Invention
This invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery comprising a positive electrode comprising a positive electrode active material composed of a reversible lithium intercalation material, a negative electrode comprising an active material composed of a reversible lithium intercalation material, and a separator interposed therebetween.
(2) Description of the Prior Art
In recent years, lithium secondary batteries have drawn considerable attention as a secondary battery capable of achieving high energy density and high power density. For the lithium secondary batteries, LiNiO2, LiCoO2, and the like have generally been used as a positive electrode active material thereof.
The lithium secondary batteries have been produced over a wide range of sizes, and among them, large-sized lithium secondary batteries are susceptible to such drawbacks that a battery temperature is raised during charge and discharge and a cycle characteristic is thereby deteriorated. This is due to the following reasons. While small-sized lithium secondary batteries retain a certain battery surface area per unit volume of power generating element, and a heat releasing quality thereof is not very much decreased. On the other hand, in the large-sized lithium secondary batteries, the battery surface area per unit volume of power generating element is rendered extremely small, and therefore the heat releasing quality is considerably deteriorated.
In relation to the above, a study by the present inventors has confirmed that there are two causes for the heat generated during charge and discharge, one being a heat generated by an internal resistance of the battery, and the other being a heat generated by an entropy change.
In view of the above, it is an object of the present invention to provide a lithium secondary battery capable of suppressing an increase of battery temperatures during charge and discharge by curbing the heat generated by an entropy change, and thereby achieving a remarkably improved cycle characteristic.
This and other objects are accomplished, in accordance with a first aspect the present invention, by providing a lithium secondary battery comprising:
a positive electrode having a positive electrode active material composed of a reversible lithium intercalation material;
a negative electrode having a negative electrode active material composed of a reversible lithium intercalation material; and
a separator interposed between the positive electrode and the negative electrode,
the positive electrode active material being a mixture in which at least one material which shows one of positive and negative signs of a total sum of entropy heat from a discharged state to a charged state is mixed with at least one material which shows the other one of the signs of the total sum of entropy heat from a discharged state to a charged state.
The following are the reasons for accomplishing the above-mentioned object.
In a battery produced in accordance with the above construction, while the battery being charged, an exothermic reaction occurs in the case where a material which shows a positive sign of the total sum of entropy heat is employed, whereas an endothermic reaction occurs in the case where a material which shows a negative sign of the total sum of entropy heat is employed. Therefore, during charge of such a battery, the amount of entropy heat is reduced in comparison with a battery employing only the material which shows a positive sign of the total sum of entropy. On the other hand, while the battery being discharged, an exothermic reaction occurs in the case where a material which shows a negative sign of the total sum of entropy heat is employed, whereas an endothermic reaction occurs in the case where a material which shows a positive sign of the total sum of entropy heat is employed. Therefore, during discharge of such a battery, the amount of entropy heat is reduced in comparison with a battery employing only the material which shows a negative sign of entropy heat.
This indicates that, in a battery employing either exclusively a material which shows a positive sign of entropy heat or exclusively a material which shows a negative sign of entropy heat, the battery temperature is increased either during charge or during discharge, and thereby the cell is conspicuously degraded and the cycle characteristic is deteriorated, On the contrary, the battery made in accordance with the above construction can suppress the increase of the battery temperature both during charge and discharge. Therefore such an undesirable effect as the degradation of the battery is avoided, and the cycle characteristic can be increased.
In accordance with a second aspect of the present invention, there is provided a lithium secondary battery comprising:
a positive electrode having a positive electrode active material composed of a reversible lithium intercalation material;
a negative electrode having a negative electrode active material composed of a reversible lithium intercalation material; and
a separator interposed between the positive electrode and the negative electrode,
the positive electrode active material being a mixture in which at least one material which shows one of positive and negative signs of entropy heat during charge at a potential of 3.0 V is mixed with at least one material which shows the other one of the signs of entropy heat during charge at the potential of 3.0 V, the potential being determined with reference to a potential of lithium metal at a temperature of 25xc2x0 C.
As in this aspect of the invention, when the materials each showing a different sign of entropy heat during charge at a discharge potential (3.0 V) are mixed, the amount of entropy heat is reduced for the same reasons as described above, and therefore the cycle characteristic is improved.
Furthermore, as a third aspect of the invention, in a lithium secondary battery according to the present invention, the positive electrode active material may be a mixture in which at least one material which shows one of positive and negative signs of entropy heat during charge in a potential of more than 3.0 V and less than 4.2 V is mixed with at least one material which shows the other one of the signs of entropy heat during charge in the potential of more than 3.0 V and less than 4.2 V, the potential being determined with reference to a potential of lithium metal at a temperature of 25xc2x0 C.
As in this construction of the invention, when the materials each showing a different sign of entropy heat during charge at an intermediate potential between a discharge potential and a charge potential (more than 3.0 V and less than 4.2 V) are mixed, the amount of entropy heat is reduced for the same reasons as described above, and therefore the cycle characteristic is improved.
Furthermore, in a lithium secondary battery according to the present invention, the positive electrode active material may be a mixture in which at least one material which shows one of positive and negative signs of entropy heat during charge at a potential of 4.2 V is mixed with at least one material which shows the other one of the signs of entropy heat during charge in the potential of 4.2 V, the potential being determined with reference to a potential of lithium metal at a temperature of 25xc2x0 C.
As in the above construction of the invention, when the materials each showing a different sign of entropy heat during charge at a charge potential (4.2 V) are mixed, the amount of entropy heat is reduced for the same reasons as described above, and therefore the cycle characteristic is improved. It is considered that in a battery made in accordance with this construction, i.e., in a battery in which two types of materials each which shows a different sign of entropy heat during charge over a potential range of from 3.0 V to 4.2 V are used, each of the materials also shows a sign of the total sum of entropy heat different from each other.
Furthermore, in a lithium secondary battery according to the present invention, a weight ratio of the material which shows one of positive and negative signs of entropy heat during charge to the material which shows the other one of the signs of entropy heat during charge may be restricted within the range of from 1:9 to 9:1.
The reason for the restriction of the weight ratio of the two types of materials as above is that the amount of entropy heat is increased during discharge in the case of an excessive amount of the material showing a negative sign of entropy heat being used, and the amount of entropy heat is increased during charge in the case of an excessive amount of the material showing a positive sign of entropy heat being used.
Furthermore, in a lithium secondary battery according to the second aspect of the present invention, a material which shows a negative sign of entropy heat during charge may consist of LiCoO2 and/or LiCo0.5Ni0.5O2, and a material which shows positive sign of entropy heat during charge may consist of LiNiO2 and/or LiMn2O4.
Furthermore, in a lithium secondary battery according to the third and fourth aspects of the present invention, a material which shows a negative sign of entropy heat during charge may consist of LiCoO2, and the material which shows positive sign of entropy heat during charge may consist of LiNiO2 and/or LiMn2O4.
Further, in a lithium secondary battery according to the present invention, a battery capacity may be made not less than 3 Ah.
For such a battery with such a large battery capacity as the one with over 3 Ah, the present invention is particularly effective, since, in such a battery, a battery surface area per unit volume of the power generating element is rendered considerably small and thereby the heat releasing quality is notably deteriorated.